This invention relates to a servo arrangement for the fine positioning of a read/write (R/W) head over the center of a data track in a magnetic disc storage system.
In such a system one or more magnetic storage discs are driven at a constant rotational speed on the order of 3,000 rpm, and each disc carries a plurality of circular, concentrically disposed data tracks spaced radially outwardly from the center of the disc. One or more R/W heads are associate with each disc, and are slidably mounted on a carriage such that they are disposed in close proximity to the disc surface. The heads are radially and very rapidly, i.e. substantially instantaneously, driven by an actuator to step them between the different data tracks, i.e. to radially jump from a present track to an addressed track in a step-like manner. The actuator and its control circuitry are preprogrammed to implement the coarse positioning of the head over the center of the addressed data track during each jump.
A conventional system for implementing the fine or more accurate centering positioning of the head(s) following each track jump is schematically shown in FIG. 1, and some of the signals generated thereby are illustrated in FIG. 2. This system generally corresponds to that described in greater detail in U.S. Pat. No. Re. 32,075.
Referring to FIG. 1, a plurality of magnetic storage discs 1 are clamped to a rotationally driven spindle 2, and each disc is associated with upper and lower R/W heads 3 radially stepped by an actuator 4 to access an addressed data track with coarse centering positioning. A data masked, sector shaped region occupying perhaps 10.degree.-15.degree. of the disc circumference is provided on each disc surface, and servo control signals to implement the fine centering positioning of the head are recorded in this sector region. The servo signals flank each data track and are circumferentially staggered or spaced such that the R/W head reads or "sees" them in quick succession.
Referring additionally to the signal diagrams shown in FIG. 2, the discs are positionally clamped to the spindles such that at the time instant when a head 3 is radially jumped by the actuator 4 from data track DT1 to DT2, for example, in response to a command signal originating in a host computer (not shown), a pickup 5 senses an index mark on the outer periphery of the spindle base and generates an index signal as shown at A in FIG. 2. The index signal is fed to a sampling counter 6 and to a servo controlling microprocessor 12. In response to the receipt of the index signal the counter 6 generates a servo period gate signal D which is applied to a sample and hold circuit 9 as an enabling signal. At he same time, the R/W head 3 (referred to in the singular for simplicity) reads the servo signals B and C flanking track DT2 in succession, and the head output is applied to the sample and hold circuit via amplifier 7 and peak detector 8. Assuming that the head is not precisely centered over the accessed data track DT2, the sampled signals resulting from the servo recordings B, C separately held in circuit 9 will have unequal amplitudes as shown at E1 and E2 in FIG. 2. These signals are applied to a differential comparator 10 whose output, representing the amplitude difference between them and thus the off-center degree or error distance of the head 3, is converted to digital form in an A/D circuit 11 and applied to the microprocessor. The polarity of the comparator output indicates the direction of the required centering correction in the usual manner. The microprocessor then generates a digital correction signal which is converted to analog form in circuit 13 and applied to a driver 14 to appropriately control the actuator 4 in a servo loop manner to accurately center the head 3 over track DT2. Such centering correction is accomplished in the very short time interval between the trailing edge of the second servo signal E2 and the rise of the gate signal D.
Of course, if the E1 and E2 signals generated by the head 3 are equal in amplitude, then the head is already precisely centered and no fin adjustment is necessary.
When the index signal or pulse of waveform A generated by pickup 5 is applied to the microprocessor 12 at the beginning of each data track cycle a software sequence is initiated in the microprocessor which generates the pulse signal shown in waveform F at the end of the servo correction. This pulse signal is delivered to the host computer via an R/W interface 15 as an R/W enable signal. In so many words, it tells the host computer that the servo correction of the head position has been completed such that read/write operations may now be commenced on data track DT2.
A recurrent problem with this prior art arrangement is that the generation of the servo period completion pulse signal F by the mentioned software sequence in the microprocessor is generally unreliable. Thus, such pulse signal often fails to appear at its specified or programmed time, which either delays the read/write sequence on the accessed data track during a given rotational cycle, which of course can result in data loss or the lack of full data recovery, or which requires the "wasting" of a full rotational cycle before the instructed read/write sequence can be initiated.